Surface acoustic wave filter device and transducer therefor

ABSTRACT

A surface acoustic wave filter device suitable for CDMA communication system includes a piezoelectric substrate, and a unidirectional input side transducer and a bidirectional output side transducer are formed on the substrate. The filter device has satisfactory filtering characteristics in terms of insertion loss, frequency characteristics, T.T.E. attenuation level, group delay time, etc.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface acoustic wave (SAW) filterdevice, in particular a surface acoustic wave filter device which issuitable for a CDMA (Code Division Multiple Access) communicationsystems, and also to a transducer which can be suitably used for such asurface acoustic wave filter device.

2. Description of the Related Art

With a recent progressive development in digital communicationtechnology, there have been proposed various communication systems. Forexample, FDMA (Frequency Division Multiple Access) system and TDMA (TimeDivision Multiple Access) system are used for communication purposes,wherein frequency band or time band is divided and assigned to eachstation. On the other hand, the CDMA system makes use of superimposedsignals in terms of frequency and time, and is recognized to be highlyuseful in view of the possibility of preserving a large number ofchannels. Thus, there is a strong demand for a further development inthe CDMA system. In this connection, various requirements in terms offiltering characteristics are imposed on surface acoustic wave filterdevices for the CDMA communication system, which may be summarized asfollows.

(a) That the filter device has an insertion loss which is not greaterthan 10 dB.

(b) That the filter device has a frequency characteristic which is freefrom distortion over a wide range.

(c) That the filter device satisfies a T.T.E. (Triple Transient Echo)attenuation level which is not less than 30 dB.

As a surface acoustic wave filter device satisfying these requirements,one may think of using a transversal-type surface acoustic wave filterdevice which includes a piezoelectric substrate, as well as abidirectional input side transducer and a bidirectional output sidetransducer, both formed on the substrate. That is to say, because abidirectional transducer has a frequency characteristic which issubstantially free from distortion, it would be possible to realize awide range filter device having a frequency characteristic which issubstantially free from distortion over a wide range, by combining twobidirectional transducers as the input side and output side transducers.

The surface acoustic wave filter device in which two bidirectionaltransducers are used as the input side and output side transducersproved to be advantageous in terms of its frequency characteristic whichis substantially free from distortion over a wide range. On the otherhand, however, when the surface acoustic wave filter device isconstituted as a transversal-type filter device, it suffers from asignificant insertion loss because surface acoustic waves excited by theinput side transducer propagate in both directions. Furthermore, it isdifficult to use bidirectional transducers as a filter device for CDMAcommunication systems because of an excessively high T.T.E. attenuationlevel due to a symmetrical electrode arrangement. In this instance, itwould be possible to attenuate the T.T.E. level by 30 dB or more, by animpedance mismatching of the filter device. However, such an impedancemismatching would result in further increase in the insertion loss. Inother words, it has been believed that a T.T.E. attenuation level of 30dB or more cannot be achieved without impedance mismatching of thetransducers and resultant increase in the insertion loss. For thesegrounds, there have been no surface acoustic wave filter device whichcan be practically used for a CDMA communication system.

SUMMARY OF THE INVENTION

It is therefore a primary object of the present invention to provide asurface acoustic wave filter device having a satisfactory filteringcharacteristics in terms of insertion loss, frequency characteristic andT.T.E. attenuation level, etc.

It is another object of the present invention to provide a transducerwhich can be suitably used for such a surface acoustic wave filterdevice.

According to one aspect of the present invention, there is provided asurface acoustic wave filter device comprising:

A) a piezoelectric substrate;

B) an input side transducer formed on said substrate, for exciting asurface acoustic wave; and

C) an output side transducer formed on said substrate, for receiving andconverting the surface acoustic wave excited by said input sidetransducer;

D) one of said transducers comprising a unidirectional transducer andthe other of said transducers comprising a bidirectional transducer.

With the above-mentioned arrangement according to the present invention,the surface acoustic wave filter device satisfies the desired filteringcharacteristic by virtue of a unique combination of transducers havingdifferent conversion characteristics. More particularly, whentransducers having different conversion characteristics are used asinput side and output side transducers of a transversal-type surfaceacoustic wave filter device, the filter device as a whole exhibits acharacteristic which corresponds to the composite characteristics of thetwo transducers. Thus, it is possible to readily realize a surfaceacoustic wave filter device having a desired filtering characteristic,by approximately combining suitable transducers in consideration oftheir proper characteristics, such as insertion loss, frequencycharacteristic, T.T.E. attenuation level, simplicity in weighting of theelectrode structure, etc.

Bidirectional transducers are known in the art as a transducer whichprovides a frequency characteristic that is substantially free fromdistortion over a wide range. Such a distortion-free frequencycharacteristic can be obtained due to a symmetrical electrodearrangement of the bidirectional transducer. Notwithstanding such anadvantageous aspect, it is a general recognition in the art thatbidirectional transducers are not very suitable for a filter device forcommunication purposes, in view of a relatively high insertion loss dueto propagation of excited surface acoustic waves in both directions.

On the other hand, unidirectional transducers are known in the art as atransducer having a low insertion loss and a high T.T.E. attenuationlevel. The unidirectional transducer has asymmetrical electrodestructure whereby a majority of energy of the excited surface acousticwave in one direction only, so that it is possible readily to minimizethe insertion loss and achieve a significantly high T.T.E. attenuationlevel. On the other hand, the asymmetrical electrode structure ofunidirectional transducers tends to cause an excessive waveformdistortion in the frequency characteristic. Thus, it is also a generalrecognition in the art that unidirectional transducers are not verysuitable for a filter device on which strict requirements are imposed onthe frequency characteristic.

The present invention is based on a novel conception that it is possibleto realize an improved surface acoustic wave filter device having asatisfactory filtering characteristics in terms of insertion loss,frequency characteristic and T.T.E. attenuation level, etc., by means ofa unique combination of a bidirectional transducer and a unidirectionaltransducer in a transversal-type filter device, so as to fully make useof a satisfactory frequency characteristic of the bidirectionaltransducer and of a low insertion loss and a high T.T.E. attenuationlevel of the unidirectional transducer.

In order to achieve a desired frequency characteristic of the surfaceacoustic wave filter device, it is a conventional practice to applyweighting of the electrodes of the transducers. When weighting is to beapplied to the electrodes of the unidirectional transducer, acomplicated process would be involved because the unidirectionalpropagation characteristic of the transducer is enhanced by phaseinversion of the surface acoustic wave at the edges of the electrodes.Thus, in a preferred embodiment of the present invention, theundirectional transducer is of a normal (i.e., unweighted) electrodestructure and only the bidirectional transducer is of a weightedelectrode structure. Such a unique combination of the two types oftransducers makes it possible readily to achieve a desired frequencycharacteristic of the surface acoustic wave filter device, by applyingweighting of the transducer electrodes in a simplified manner.

According to a particularly advantageous embodiment of the surfaceacoustic wave filter device of the present invention, the unidirectionaltransducer comprises (i) a positive electrode comprising a plurality ofelectrode fingers which are periodically arranged at a pitch λ that is apropagation wavelength of a fundamental surface acoustic wave, (ii) anegative electrode comprising a plurality of electrode fingers which areperiodically arranged at said pitch λ and each situated with respect toan adjacent pair of said electrode fingers of the positive electrode ata center distance λ/2, and (iii) a floating electrode comprising aplurality of electrode fingers each situated between an adjacentelectrode finger of said positive electrode and an adjacent electrodefinger of said negative electrode, and offset from a intermediateposition between said adjacent electrode fingers of said positive andnegative electrodes, in a direction opposite to a propagation directionof said surface acoustic wave in the case of the input side transducer,and in the propagation direction of said surface acoustic wave in thecase of the output side transducer, (iv) each of said electrode fingersof the positive and negative electrodes and of said floating electrodeof the unidirectional transducer having a width λ/12 as measured in saidpropagation direction of said surface acoustic wave, and (v) eachelectrode finger of said floating electrode of the unidirectionaltransducer being offset from said intermediate position by a distanceλ/12.

Furthermore, the bidirectional transducer comprises (vi) a positiveelectrode comprising a plurality of sets of two electrode fingers whichare spaced from each other at a center distance λ/4, each electrodefinger of said sets of the positive electrode having a width λ/8 asmeasured in said propagation direction of said surface acoustic wave,and said sets of electrode fingers of the positive electrode beingperiodically arranged at a pitch λ, and (vii) a negative electrodecomprising a plurality of sets of two electrode fingers which are spacedfrom each other at a center distance λ/4, each electrode finger of saidsets of the negative electrode having a width λ/8 as measured in saidpropagation direction of said surface acoustic wave, and each set ofelectrode fingers of the negative electrode being arranged betweenadjacent sets of electrode fingers of the positive electrode.

By using the above-mentioned unidirectional transducer, it is possibleto arrange each floating electrode finger at a location which issignificantly offset from the intermediate position between adjacentelectrode fingers of the positive and negative electrodes, in adirection which is opposite to the propagation direction of the surfaceacoustic wave. Such a unique offset arrangement of the floatingelectrode finger serves to enhance the unidirectional propagationcharacteristic of the transducer and thereby reduce the insertion lossof the filter device.

In this connection, each electrode finger of the positive and negativeelectrodes and of the floating electrode has a width λ/12 measured inthe propagation direction of the surface acoustic wave, and the offsetamount of the floating electrode is λ/12. Thorough investigationscarried out by the inventors revealed that, in many cases, asatisfactory unidirectional propagation characteristic of the transducerdue to the asymmetrical structure of the transducer cannot be readilyachieved when the offset amount is relatively small e.g., λ/8. Theelectrode finger width of λ/12 in combination with the offset amount ofλ/12 makes it possible to regularly and orderly arrange the electrodefingers of the positive and negative electrodes and of the floatingelectrodes at a basic pitch of λ/12.

Furthermore, for effectively realizing a satisfactory frequencycharacteristic with a minimized distortion, each electrode finger of thebidirectional transducer is of a so-called split structure and has awidth λ/8. Such an electrode arrangement of the bidirectional transducerserves effectively to cancel surface acoustic waves reflected at theedges of the electrodes. Thus, it is possible to reduce the reflectedwaves which are different in phase, and thereby lower the ripples andimprove the insertion loss characteristic of the filter device.

According to another preferred embodiment of the present invention, thepiezoelectric substrate comprises a quartz substrate and the floatingelectrode comprises a shortcircuit-type electrode.

As known in the art, quartz exhibits only a small change in frequencyrelative to temperature variation. Thus, when the piezoelectricsubstrate is comprised of quartz, it is possible to maintain variationof frequency pass band range of the filter device due to temperaturerange within an allowable range. However, because quartz has a smallelectro-mechanical coupling coefficient, a surface acoustic wave filterdevice comprising a quartz substrate on which are formed knowntransducers as they are cannot be practically used due to a significantinsertion loss of the device.

As a result of thorough investigations carried out by the inventors withreference to the insertion loss of a quartz substrate, it has beenrevealed that the sign of the reflection coefficient of the floatingelectrode is highly influential over the insertion loss. Thus, in thecase of a quartz substrate, shortcircuit-type floating electrode has areflection coefficient which is higher than that of open-type floatingelectrode, and it is therefore preferable to form the floating electrodeas a shortcircuit-type floating electrode. In this instance, it ispossible to minimize the insertion loss of the filter device even whenuse is made of a quartz substrate having a small electro-mechanicalcoupling coefficient, thereby realizing a wide band surface acousticwave filter device having an improved temperature characteristic and aminimized insertion loss.

According to another preferred embodiment of the present invention, theunidirectional transducer comprises at least one electrode comprising aplurality of sets of first and second electrodes, said sets of electrodefingers being periodically arranged at a pitch λ which is thepropagation wavelength of said surface acoustic wave, said firstelectrode finger having a width λ/8, said second electrode finger havinga width 3λ/8, and said first and second electrode fingers being spaced adistance 3λ/8.

The above-mentioned combination of the electrode finger having a widthλ/8 with the electrode finger having a width 3λ/8, the surface acousticwaves incident on the unidirectional transducer are reflected due tomismatching of acoustic impedance at the edges of the respectiveelectrode fingers. However, the surface acoustic wave reflected at theedges of an electrode finger toward the direction opposite to thepropagation direction of surface acoustic wave has a phase difference ofλ/2 relative to the surface acoustic wave excited at the edge of theadjacent electrode finger and propagating in the normal propagationdirection, so that undesirable reflected waves are substantiallycompletely cancelled without the sacrifice of the insertion loss.

According to still another embodiment of the present invention, theunidirectional transducer comprises:

A) a first electrode which comprises a plurality of sets of first andsecond electrode fingers, which sets are periodically arranged at apredetermined pitch, said first and second electrode fingers of each sethaving a width λ/4 as measured in the propagation direction of thesurface acoustic wave, where λ is the propagation wavelength of saidsurface acoustic wave, and spaced from the other by a center distance λ;

B) a second electrode comprising a plurality of electrode fingers eacharranged between the first and second electrode fingers of each set ofsaid first electrode, and having a width λ/4; and

C) at least one floating electrode arranged between adjacent first andsecond electrode fingers of said first electrode and having a width λ/4.

In this instance, surface acoustic waves propagating in a directionopposite to the normal propagation direction are reflected by thefloating electrode arranged between adjacent electrode fingers of thefirst and second electrodes, so that it is possible to realize a surfaceacoustic wave filter device having a further reduced insertion loss.

According to still another embodiment of the present invention, theunidirectional transducer comprises:

A) a first electrode comprising a plurality of sets of electrode fingersarranged at a predetermined pitch, each of said sets of the firstelectrode comprising first, second, third and fourth electrode fingerseach having a width λ/8 as measured in the propagation direction of thesurface acoustic wave, where λ is the propagation wavelength of saidsurface acoustic wave, said first and second electrode fingers of eachset being spaced from each other by a center distance λ/4, said secondand third electrode fingers of each set being spaced from each other bya center distance 3λ/4, and said third and fourth electrode fingers ofeach set being spaced from each other by a center distance λ/4;

B) a second electrode comprising a plurality of sets of electrodefingers arranged at a predetermined pitch, each of said sets of thesecond electrode comprising fifth and sixth electrode fingers eachhaving a width λ/8 as measured in the propagation direction of thesurface acoustic wave, said fifth and sixth electrode fingers of eachset being spaced from each other by a center distance λ/4 and arrangedbetween the first and second electrode fingers of an adjacent set of thefirst electrode; and

C) at least one floating electrode having a width λ/4 as measured in thepropagation direction of the surface acoustic wave and arranged betweenneighbouring sets of electrode fingers of the first electrode.

In this instance, surface acoustic waves propagating in a directionopposite to the normal propagation direction are reflected by thefloating electrodes arranged between adjacent sets of electrode fingersof the first electrode, so that it is possible to realize a surfaceacoustic wave filter device having a further reduced insertion loss.

The present invention also provides a transducer which is suitable for asurface acoustic wave filter device. The transducer according to thepresent invention comprises an interdigital-type transducer including apiezoelectric substrate, a positive electrode having a plurality ofelectrode fingers, and a negative electrode having a plurality ofelectrode fingers each arranged between adjacent electrode fingers ofthe positive electrode, said positive and negative electrodes beingformed on the substrate, wherein the transducer is comprised of aunidirectional transducer portion and a bidirectional transducerportion.

When weighting is applied to the transducer by changing the overlappinglength of adjacent electrode fingers in the propagation direction of thesurface acoustic wave, the influence of refraction of the surfaceacoustic wave becomes more significant and the insertion loss of thedevice deteriorates as the overlapping length decreases. By forming partof the transducer as a unidirectional transducer portion, in thetransducer according to the present invention, it is possible to improvethe insertion loss while maintaining the overall characteristics of thetransducer.

According to still another embodiment of the present invention, thetransducer is of a weighted electrode type and includes electrodefingers having a maximum overlapping length with reference to adjacentelectrode finger, and that portion of said substrate in which saidelectrode fingers having a maximum overlapping length are arranged isformed as a unidirectional transducer portion. Such an arrangement ofthe transducer also serves to improve the insertion loss whilemaintaining the overall characteristics of the transducer.

According to still another embodiment of the present invention, thesurface acoustic wave filter device comprises a piezoelectric substrate,an input side transducer having a bidirectional electrode structure, anda pair of output side transducers on both sides of the input sidetransducer and each having a unidirectional electrode structure. In thisinstance, surface acoustic waves excited by the bidirectional input sidetransducer are propagated in both directions and received by theunidirectional output side transducers, so as to further reduce theinsertion loss.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view showing a surface acoustic wave filterdevice according to a first embodiment of the present invention;

FIG. 2 is a schematic plan view showing a surface acoustic wave filterdevice according to a second embodiment of the present invention;

FIG. 3 is a schematic plan view showing a surface acoustic wave filterdevice according to a third embodiment of the present invention;

FIG. 4 is a schematic plan view showing one embodiment of transducer forsurface acoustic wave filter device according to the present invention;

FIG. 5 is a schematic plan view showing a surface acoustic wave filterdevice according to a fourth embodiment of the present invention;

FIG. 6 is a schematic plan view showing a surface acoustic wave filterdevice according to a fifth embodiment of the present invention;

FIG. 7 is a schematic plan view showing a surface acoustic wave filterdevice according to a sixth embodiment of the present invention;

FIG. 8 is a schematic plan view showing a surface acoustic wave filterdevice according to a seventh embodiment of the present invention;

FIG. 9 is a graph showing the filtering characteristic of the filterdevice according to the embodiment of FIG. 1;

FIG. 10 is a graph showing the filtering characteristic of a comparativefilter device in which the bidirectional transducer in the embodiment ofFIG. 1 has been replaced by a unidirectional transducer;

FIG. 11 is a graph showing the filtering characteristic of the filterdevice according to the embodiment of FIG. 5; and

FIG. 12 is a graph showing the filtering characteristic of the filterdevice according to the embodiment of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be further explained below with reference tosome preferred embodiments shown in the accompanying drawings.

FIG. 1 is a schematic plan view of the surface acoustic wave filterdevice according to a first embodiment of the present invention. Thesurface acoustic wave filter device of this embodiment includes agenerally rectangular piezoelectric substrate 1 which is comprised ofquartz. As known in the art, when used as a substrate of surfaceacoustic wave filter devices, quartz exhibits a small variation in thepass band width relative to temperature variation and thus serves tominimize the fluctuation of the frequency characteristic due totemperature variation. The substrate 1 has a surface provided thereonwith an input side transducer 2, a shield electrode 3 and an output sidetransducer 4 which are arranged in the stated sequence along apropagation axis of the surface acoustic wave excited by the input sidetransducer 2 and received by the output side transducer 4.

The input side transducer 2 is a unidirectional transducer which iscomprised of a positive electrode 5, a negative electrode 6 and aplurality of floating electrodes 7, each having a plurality of electrodefingers. Each electrode finger of the positive and negative electrodes5, 6 and the floating electrodes 7 has a width as measured in thepropagation direction of the surface acoustic wave, which is 1.1×λ/12,where λ is the wavelength of the fundamental surface acoustic wave.Adjacent electrode fingers of the positive electrode 5 are spaced fromeach other and periodically arranged at a pitch λ. Similarly, adjacentelectrode fingers of the negative electrode 6 are spaced from each otherand periodically arranged at a pitch λ. Each electrode finger of thenegative electrode 6 is arranged between adjacent electrode fingers ofthe positive electrode 5 in an interdigital manner. The center distancebetween each electrode finger of the positive electrode 5 and anadjacent electrode finger of the negative electrode 6 is λ/2. Eachelectrode finger of the floating electrodes 7 is arranged at a locationwhich is offset by an amount of λ/12, from the intermediate positionbetween adjacent electrode fingers of the positive and negativeelectrodes 5 and 6, toward the upstream side of the propagationdirection of the surface acoustic wave, thereby to enhance theunidirectional characteristic of the transducer due to an asymmetricalstructure. As for the input side transducer 2, the number of pairs ofpositive and negative electrode fingers may be determined asappropriate, depending upon the desired filtering characteristic. Forexample, there may be provided forty pairs of the electrode fingers.

In the embodiment shown in FIG. 1, each electrode finger of the floatingelectrodes 7 is substantially offset from the intermediate positionbetween adjacent electrode fingers of the positive and negativeelectrodes 5 and 6 to provide an enhanced unidirectional characteristicof the transducer due to asymmetrical electrode finger arrangement,which serves effectively to lower the insertion loss of the filterdevice. A unidirectional transducer with such asymmetrical electrodefinger arrangement is known, per se, in which the electrode finger widthis set to be λ/8. In this instance, however, it is often difficulteffectively to realize a satisfactory unidirectional characteristic ofthe transducer, because each electrode finger of the floating electrodescould not be sufficiently offset from the intermediate position betweenadjacent electrode fingers of the positive and negative electrodes.Therefore, according to the present invention, each electrode finger ofthe floating electrodes 7 has a width λ/12 and is offset by an amount ofλ/12 from the intermediate position between adjacent electrode fingersof the positive and negative electrodes 5 and 6, toward the upstreamside of the propagation direction of the surface acoustic wave. Thismeans that the present invention makes it possible to regularly arrangethe electrode fingers of the positive and negative electrodes and of thefloating electrodes, at a basic pitch of λ/12. Moreover, the electrodefinger arrangement according to the present invention serves to minimizethe ripple, and this is due to the fact that occurrence of irregularphase shifting as a result of reflection of surface acoustic waves atthe edges of the electrode fingers does not take place.

The output side transducer 4 includes a positive electrode 8 and anegative electrode 9 each having a split electrode structure in which aplurality of sets of two electrode fingers are periodically arranged ata pitch λ, and the two electrode fingers of each set are spaced from theother at a center distance of λ/4. In order to provide an improvedconversion efficiency of the surface acoustic wave, each electrodefinger of the positive electrode 8 and the negative electrode 9 has awidth which is λ/8. As for the output side transducer 4, the number ofsets of positive and negative electrode fingers may be determined asappropriate, depending upon the desired characteristic. For example,there may be provided 300 sets of electrode fingers of the positiveelectrode 8 and the negative electrode 9.

The output side transducer 4 has an interdigital-type electrode fingerarrangement wherein the positive and negative electrode fingers whichare overlapped with each other as seen in the propagation direction ofthe surface acoustic wave. Moreover, the electrode finger arrangement ofthe output side transducer 4 is weighted in accordance with theso-called apodizing method, wherein the overlapping length of thepositive and negative electrode fingers as measured in a directionperpendicular to the propagation direction of the surface acoustic wavevaries in the propagation direction of the surface acoustic wave.

The operation of the surface acoustic wave filter device according tothe embodiment of FIG. 1 will be explained below.

When an input electric signal to be filtered is applied to inputterminals 10, 11 connected respectively to the positive electrode 5 andthe negative electrode 6 of the input side transducer 2, surfaceacoustic waves are excited by the input side transducer 2 and propagatesubstantially in one direction only, toward the output side transducer4, via the shield electrode 3. The surface acoustic waves are convertedby the output side transducer 4 into an electric signal which is thensupplied to output terminals 12, 13 of the filter device which areconnected respectively to the positive electrode 8 and the negativeelectrode 9 of the output side transducer 4. The output signal suppliedto the output terminals 12, 13 has a frequency characteristic whichcorresponds to the frequency characteristic of the input side transducer2 as multiplied by that of the output side transducer 4. Thus, by meansof an optimum combination of the input side and output side transducersin a transversal-type filter device, it is possible to realize a surfaceacoustic wave filter device which fully makes use of a satisfactoryfrequency characteristic of of the output side transducer 4 incombination with advantageous characteristics of the input sidetransducer in terms of insertion loss and T.T.E. (Triple Transient Echo)level. Therefore, according to the embodiment of FIG. 1, it is possibleto realize a surface acoustic wave filter device which is capable ofsatisfying all the requirements for the frequency characteristic,insertion loss and T.T.E. level.

Similar advantageous functions can also be achieved when the input sidetransducer is comprised of a bidirectional transducer and combined withan output side transducer which, in turn, is comprised of aunidirectional transducer. Also, while the electrode 8 and the electrode9 have been used as a positive electrode and a negative electrode,respectively, the relationship may be reversed so that the electrode 8and the electrode 9 are used as a negative electrode and a positiveelectrode, respectively.

FIG. 2 is a schematic plan view of the surface acoustic wave filterdevice according to a second embodiment of the present invention. Thesurface acoustic wave filter device of this embodiment includes apiezoelectric substrate 21, as well as an input side transducer 22, ashield electrode 23 and an output side transducer 24 which are formed onthe substrate as in the previous embodiment.

The input side transducer 22 of this embodiment is a unidirectionaltransducer including a positive electrode 25 and a negative electrode26. The positive electrode 25 is comprised of a plurality of electrodefingers 25a each having a width λ/8, which are periodically arranged ata pitch λ, i.e., the wavelength of the fundamental surface acoustic wavegenerated by the positive electrode 25. The negative electrode 26 iscomprised of a plurality of sets of first and second electrode fingers26a, 26b, which sets are periodically arranged at a pitch λ. The firstelectrode finger 26a has a width λ/8 and the second electrode finger 26bhas a width 3λ/8. Furthermore, the first and second electrode fingers26a, 26b of a set are spaced from each other by a center distance of3λ/4. The output side transducer 24 is a bidirectional transducerincluding a positive electrode 28 and a negative electrode 29, and has astructure which is essentially the same as the output side transducer inthe embodiment of FIG. 1.

According to the embodiment of FIG. 2, when an input signal is suppliedto input terminals 30, 31 connected respectively to the positiveelectrode 25 and the negative electrode 26 of the input side transducer22 and the surface acoustic waves are excited by the input sidetransducer 22, the surface acoustic waves are reflected at the edges ofthe electrode fingers due to mis-matching of the acoustic impedance.However, due to a unique electrode finger arrangement of theunidirectional input side transducer 22 wherein the negative electrode26 includes sets of first electrode finger 26a with a width λ/8 andsecond electrode finger 26b with a width 3λ/8, it is possible tosubstantially completely cancel the reflection of the surface acousticwaves toward an opposite direction, i.e., away from the output sidetransducer 24. More specifically, the surface acoustic wave reflected atthe edge of an electrode finger toward the opposite direction has aphase difference of λ/2 relative to the surface acoustic wave excited atthe edge of an adjacent electrode finger and propagating in the normalpropagation direction, so that the undesirable reflected waves aresubstantially completely cancelled. Therefore, in consideration of thesurface acoustic wave filter device as a whole, the input sidetransducer 22 functions as if it excites surface acoustic waves onlywhich propagate exclusively in the desired direction. It should be notedthat the resultant reflected waves can be substantially completelycancelled or eliminated without the sacrifice of the insertion loss ofthe filter device. In other respects, the operation and function of theembodiment of FIG. 2 are essentially same as those of the embodiment ofFIG. 1.

Incidentally, reference numerals 32 and 33 in FIG. 2 denote outputterminals of the filter device, which are connected respectively to thepositive electrode 28 and the negative electrode 29 of the output sidetransducer 24.

FIG. 3 is a schematic plan view of the surface acoustic wave filterdevice according to a third embodiment of the present invention. Thesurface acoustic wave filter device of this embodiment includes apiezoelectric substrate 41, as well as an input side transducer 42, ashield electrode 43 and an output side transducer 44 which are formed onthe substrate 41 as in the previous embodiment.

The input side transducer 42 of this embodiment is a unidirectionaltransducer including a positive electrode 45 and a negative electrode46. The positive electrode 45 is comprised of a plurality of sets offirst and second electrode fingers 45a, 45b, which sets are periodicallyarranged at a pitch 3λ, where λ is the wavelength of the fundamentalsurface acoustic wave. The first and second electrode fingers 45a, 45beach has a width λ/4 as measured in the propagation direction of thesurface acoustic wave. The first and second electrode fingers 45a, 45bof a set are spaced from each other by a center distance λ.

The negative electrode 46 is comprised of a plurality of electrodefingers which are periodically arranged at a pitch 3λ. Each electrodefinger of the negative electrode 46 is arranged at the intermediateposition of the adjacent first and second electrode fingers 45a, 45b ofthe positive electrode 45.

Furthermore, a reflector array 47 is arranged between the adjacent setsof the first and second electrode fingers 45a, 45b of the positiveelectrode 45. The array 47 is comprised of a set of three floatingelectrodes 47a, each having a width λ/4 and being spaced from the otherby a center distance λ/4. The pitch of the sets of electrode fingers45a, 45b of the positive electrode 45 may be changed depending upon thenumber of the floating electrodes 47a.

The output side transducer 44 is a bidirectional transducer including apositive electrode 48 and a negative electrode 49, and has a structurewhich is essentially the same as the output side transducer in theembodiment of FIG. 1.

In the embodiment of FIG. 3, when an input signal is supplied to theinput terminals 50, 51 connected respectively to the positive electrode45 and negative electrode 46 of the input side transducer and thesurface acoustic waves are excited by the input side transducer 42, thesurface acoustic waves propagating in the opposite direction away fromthe output side transducer 44 are reflected by the reflector array 47,and it is thus possible to realize a surface acoustic wave filter devicehaving a minimized insertion loss. Reference numerals 52 and 53 in FIG.3 denote output terminals of the filter device, which are connectedrespectively to the positive electrode 48 and the negative electrode 49of the output side transducer 44.

The operation and function of the embodiment of FIG. 3 are essentiallythe same as those of the previous embodiments shown in FIG. 1 or FIG. 2.

In the embodiment shown in FIG. 3, the input side transducer 42 may bereplaced by a unidirectional transducer, not shown, which includes afirst electrode comprised of a plurality of sets of first, second, thirdand fourth electrode fingers, a second electrode comprised of aplurality of sets of fifth and sixth electrode fingers, and at least onefloating electrode arranged between adjacent sets of electrode fingersof the first electrode. In such a unidirectional transducer, the sets ofelectrode fingers of the first electrode are periodically arranged at apredetermined pitch. Similarly, the sets of electrode fingers of thesecond electrode are periodically arranged at a predetermined pitch.Each electrode finger of the first electrode and the second electrodehas a width λ/8. In each set of electrode fingers of the firstelectrode, the first and second electrode fingers are spaced from eachother by a center distance λ/4, the second and third electrode fingersare spaced from each other by a center distance 3λ/4, and the third andfourth electrode fingers are spaced from each other by a center distanceλ/4. Further, in each set of electrode fingers of the second electrode,the fifth and sixth electrode fingers are spaced from each other by acenter distance λ/4. Each set of fifth and sixth electrode fingers inthe second electrode is arranged between the second and third electrodefingers of a set in the first electrode.

FIG. 4 is a schematic plan view of one embodiment of a transducer for asurface acoustic wave filter device, according to the present invention.The transducer of this embodiment includes a piezoelectric substrate 61comprising quartz, as well as a pair of interdigital-type first andsecond bidirectional electrode portions 62a, 62b and a unidirectionalelectrode portion 63 which are formed on the substrate 61, wherein thebidirectional electrode portions 62a, 62b are arranged on both sides ofthe unidirectional electrode portion 63.

The first bidirectional electrode portion 62a includes a positiveelectrode 64a which is comprised of a plurality of sets of electrodefingers, and a negative electrode 65a which is also comprised of aplurality of sets of electrode fingers. The sets of electrode fingers ofthe positive electrode 64a are periodically arranged at a pitch λ, whereλ is the wavelength of the fundamental surface acoustic wave, with theelectrode fingers of each set being spaced from each other by a centerdistance λ/4. Similarly, the sets of electrode fingers of the negativeelectrode 65a are periodically arranged at a pitch λ, with the electrodefingers of each set being spaced from each other by a center distanceλ/4. Each set of electrode fingers of the negative electrode 65a isspaced from a adjacent set of electrode fingers of the positiveelectrode 64a by a center distance λ/2. The electrode fingers of thepositive electrode 64a and those of the negative electrode 65a each hasa width λ/8 as measured in the propagation direction of the surfaceacoustic wave. The electrode fingers of the positive and negativeelectrodes 64a, 65a in the first bidirectional electrode portion 62a areweighted in accordance with the apodizing method, so that theoverlapping length of adjacent electrode fingers of the positive andnegative electrodes 64a, 65a varies in the propagation direction of thesurface acoustic wave.

The second bidirectional electrode portion 62b is essentially same asthe first bidirectional electrode portion 62a explained above. Thus, thesecond bidirectional electrode portion 62b includes a positive electrode64b which is comprised of a plurality of sets of electrode fingers, anda negative electrode 65b which is also comprised of a plurality of setsof electrode fingers. The sets of electrode fingers of the positiveelectrode 64b are periodically arranged at a pitch λ, with the electrodefingers of each set being spaced from each other by a center distanceλ/4. Similarly, the sets of electrode fingers of the negative electrode65b are periodically arranged at a pitch λ, with the electrode fingersof each set being spaced from each other by a center distance λ/4. Eachset of electrode fingers of the negative electrode 65b is spaced from anadjacent set of electrode fingers of the positive electrode 64b by acenter distance λ/2. The electrode fingers of the positive electrode 64band those of the negative electrode 65b each has a width λ/8 as measuredin the propagation direction of the surface acoustic wave. The electrodefingers of the positive and negative electrodes 64b, 65b in the secondbidirectional electrode portion 62b are weighted in accordance with theapodizing method, so that the overlapping length of adjacent electrodefingers of the positive and negative electrodes 64b, 65 b varies in thepropagation direction of the surface acoustic wave.

The unidirectional electrode portion 63 includes a positive electrode64c comprised of a plurality of electrode fingers, a negative electrode65c also comprised of a plurality of electrode fingers, and floatingelectrodes 66 each being arranged between adjacent electrode fingers ofthe positive and negative electrodes 64c, 65c. The unidirectionalelectrode portion 63 is arranged between the first and secondbidirectional electrode portions 62a, 62b such that it is situatedadjacent to those electrode fingers of the two electrode portions 62a,62b having the maximum overlapping length.

FIG. 5 is a schematic plan view of a fourth embodiment of the surfaceacoustic wave filter device according to the present invention. Thefilter device of this embodiment includes a piezoelectric substrate 71comprising quartz, as well as a unidirectional input side transducer 74,a pair of bidirectional output side transducers 72a, 72b on both sidesof the input side transducer 74, and shield electrodes 73a, 73b betweenthe input side transducer 74 and output side transducers 72a, 72b, whichare formed on the substrate 71. The input side transducer 74 and outputside transducers 72a, 72b are arranged so that the respectivepropagation axes of the surface acoustic wave coincide with each other.

The input side transducer 74 includes a positive electrode 78 and anegative electrode 79, and is essentially the same as the output sidetransducer in the embodiment of FIG. 1. The input side transducer 74 inthis embodiment has 330 pairs of positive and negative electrodefingers. The output side transducer 72a includes a positive electrode75a, a negative electrode 76a and floating electrodes 77a. Similarly,the output side transducer 72b includes a positive electrode 75b, anegative electrode 76b and floating electrodes 77b. Each output sidetransducer 72a, 72b in this embodiment is essentially the same as theinput side transducer in the embodiment of FIG. 1, and has 40 pairs ofpositive and negative electrode fingers.

In FIG. 5, reference numerals 82, 83 denote the input terminals of thesurface acoustic wave filter device, and reference numerals 80a, 80b,81a, 81b denote the output terminals of the surface acoustic wave filterdevice.

According to the embodiment of FIG. 5, a pair of unidirectional outputside transducers 72a, 72b are arranged on both sides of thebidirectional input side transducer 74, and the output electric signalsfrom the terminals 80a, 80b, 81a, 81b connected to these output sidetransducers 72a, 72b are added or composited with each other to furtherimprove the insertion loss characteristic of the filter device.

FIG. 6 is a schematic plan view showing a surface acoustic wave filterdevice according to a fifth embodiment of the present invention. Thefilter device of this embodiment includes the bidirectional transduceraccording to the embodiment of FIG. 4, which is combined with aunidirectional transducer according to the embodiment of FIG. 1, Thisembodiment is advantageous in that, because the bidirectional transducerhas a unidirectional transducer portion, it is possible to further lowerthe insertion loss of the filter device by several dBs.

FIG. 7 is a schematic plan view showing a surface acoustic wave filterdevice according to a sixth embodiment of the present invention. Thefilter device of this embodiment has a structure which is basically thesame as the embodiment of FIG. 5, except that the unidirectionaltransducers are replaced by those of the embodiment of FIG. 2.

FIG. 8 is a schematic plan view showing a surface acoustic wave filterdevice according to a seventh embodiment of the present invention. Thefilter device of this embodiment has a structure which is basically thesame as the embodiment of FIG. 5, except that the unidirectionaltransducers are replaced by those of the embodiment of FIG. 3.

In the following, explanation will be made of the results of experimentsperformed with respect to the filtering characteristics of the surfaceacoustic wave filter device according to the present invention. Theseexperiments were conducted by preparing samples of the filter deviceaccording to the first, fourth and fifth embodiments shown in FIGS. 1, 5and 6, respectively. For each sample filter device, frequencycharacteristics, insertion loss, group delay time (GDT) and T.T.E.attenuation level were measured. The specification of the sample filterdevices prepared is as follows:

1. substrate: quartz

2. number of electrode pairs:

2.1) 300 pairs for bidirectional transducer

2.2) 40 pairs for undirectional transducer

3. center frequency: 85.38 MHz

4. weighting: according to the apodizing method for bidirectionaltransducer

FIG. 9 is a graph showing the filtering characteristic of the filterdevice according to the embodiment shown in FIG. 1. In FIG. 9, theabscissa shows the frequency (MHz) and the ordinate shows theattenuation level (dB) and the group delay time (GDT; μsec). The solidline corresponds to the attenuation level and the broken linecorresponds to the group delay time. It can be recognized from FIG. 9that the frequency characteristic is substantially free from ripples ordistortion within a wide pass band range on both sides of the centerfrequency, and further that the insertion loss is about 10 dB, GDTripple is about 0.2 μsec, and T.T.E. attenuation level is about 40 dB.

FIG. 10 is a graph showing the filtering characteristic of a comparativefilter device in which the bidirectional transducer in the embodiment ofFIG. 1 has been replaced by a unidirectional transducer. This means thatthe input side and output side transducers of the comparative filterdevice are both formed of unidirectional transducers. The insertion lossis lowered to about 8 dB, though ripples of the magnitude of 1 dB ormore can be recognized in the frequency characteristic near the centerfrequency. It is clear that such ripples resulted from application ofweighting to the unidirectional transducers in the comparative filterdevice. Therefore, it proved to be highly advantageous to applyweighting to the bidirectional transducer having a symmetrical electrodearrangement, and to combine such bidirectional transducer with aunidirectional transducer of a normal type which is not applied withweighting.

FIG. 11 is a graph showing the filtering characteristic of the filterdevice according to the embodiment of FIG. 5. In this case, the filterdevice has a frequency characteristic which is substantially free fromripples or distortion within a wide pass band range on both sides of thecenter frequency, an insertion loss of about 8.5 dB, GDT ripple of about0.3 μsec, and T.T.E. attenuation level of about 50 dB.

FIG. 12 is a graph showing the filtering characteristic of the filterdevice according to the embodiment of FIG. 6. In this case, the filterdevice has a frequency characteristic which is substantially free fromripples or distortion within a wide pass band range on both sides of thecenter frequency, an insertion loss of about 8 dB which is lower by 2 dBthan the insertion loss shown in FIG. 9, GDT ripple of about 0.3 μsec,and T.T.E. attenuation level of about 50 dB.

It can be clearly appreciated from FIGS. 9 to 12 that the surfaceacoustic wave filter device according to the present invention providessatisfactory filtering characteristics in terms of frequencycharacteristic, insertion loss, GDT and T.T.E. attenuation level, and itis thus particularly suitable as a filter device for CDMA communicationsystem.

While the present invention has been described above with reference tocertain preferred embodiments, it should be noted that they werepresented by way of examples only and various changes and/ormodifications may be made without departing from the scope of theinvention. Thus, for example, the substrate of the device may becomprised of LiNbO₃, LiTaO₃, LiB₄ O₇ or other suitable piezoelectricmaterial, instead of quartz as in the illustrated embodiments. Inparticular, a substrate comprising LiTaO₃ or LiB₄ O₇ haselectro-mechanical coupling coefficient and reflection coefficient whichare substantially same as those of quartz. Therefore, LiTaO₃ substrateand LiB₄ O₇ substrate each provides particularly excellent filteringcharacteristics like the quartz substrate.

We claim:
 1. A surface acoustic wave filter device comprising:apiezoelectric substrate; an input side transducer formed on saidsubstrate, for exciting a surface acoustic wave; and an output sidetransducer formed on said substrate, for receiving and converting thesurface acoustic wave excited by said input side transducer; one of saidtransducers comprising a unidirectional transducer and the other of saidtransducers comprising a bidirectional transducer, wherein: saidunidirectional transducer comprises (i) a positive electrode comprisinga plurality of electrode fingers that are periodically arranged at apitch λ that is a propagation wavelength of a fundamental surfaceacoustic wave, (ii) a negative electrode comprising a plurality ofelectrode fingers that are periodically arranged at said pitch λ andeach situated with respect to an adjacent pair of said electrode fingersof the positive electrode at a center distance λ/2, and (iii) a floatingelectrode comprising a plurality of electrode fingers each situatedbetween an adjacent one of said electrode fingers of said positiveelectrode and an adjacent one of said electrode fingers of said negativeelectrode, and offset from an intermediate position between saidadjacent electrode fingers of said positive and negative electrodes, ina direction opposite to a propagation direction of said surface acousticwave in the case of the input side transducer, and in the propagationdirection of said surface acoustic wave in the case of the output sidetransducer, (iv) each of said electrode fingers of the positive andnegative electrodes and of said floating electrode of the unidirectionaltransducer having a width λ/12 as measured in said propagation directionof said surface acoustic wave, and (v) each electrode finger of saidfloating electrode of the unidirectional transducer being offset fromsaid intermediate position by a distance λ/12; and said bidirectionaltransducer comprises (i) a positive electrode comprising a plurality ofsets of two electrode fingers that are spaced from each other by acenter-to-center distance λ/4, each electrode finger of said sets of thepositive electrode having a width λ/8 as measured in said propagationdirection of said surface acoustic wave, and said sets of electrodefingers of the positive electrode being periodically arranged at a pitchλ, and (ii) a negative electrode comprising a plurality of sets of twoelectrode fingers that are spaced from each other by a center-to-centerdistance λ/4, each electrode finger of said sets of the negativeelectrode having a width λ/8 as measured in said propagation directionof said surface acoustic wave, and each set of electrode fingers of thenegative electrode being arranged between adjacent sets of electrodefingers of the positive electrode.
 2. The surface acoustic wave filterdevice according to claim 1, wherein said piezoelectric substratecomprises a quartz substrate and said floating electrode comprises ashortcircuit-type electrode.
 3. The surface acoustic wave filter deviceaccording to claim 1, wherein said piezoelectric substrate comprisesLiTaO₃.
 4. The surface acoustic wave filter device according to claim 1,wherein said unidirectional transducer is of normal electrode type andsaid bidirectional transducer is of a weighted electrode type.
 5. Thesurface acoustic wave filter device according to claim 4, wherein saidbidirectional transducer of a weighted electrode type comprises saidelectrode fingers of said positive and negative electrodes havinglengths measured in a direction perpendicular to a propagation directionof the surface acoustic wave, said lengths of the electrode fingersvarying gradually in the propagation direction of the surface acousticwave in accordance with the apodization-method.
 6. A surface acousticwave filter device for a CDMA communication system, comprising:apiezoelectric substrate; an input side transducer formed on saidsubstrate, for exciting a surface acoustic wave; and an output sidetransducer formed on said substrate, for receiving and converting thesurface acoustic wave excited by said input side transducer; one of saidtransducers comprising a unidirectional transducer and the other of saidtransducers comprising a bidirectional transducer; said surface acousticwave filter device having a T.T.E. attenuation level not less than 30dB, wherein: said unidirectional transducer comprises (i) a positiveelectrode comprising a plurality of electrode fingers that areperiodically arranged at a pitch λ that is a propagation wavelength of afundamental surface acoustic wave, (ii) a negative electrode comprisinga plurality of electrode fingers that are periodically arranged at saidpitch λ and each situated with respect to an adjacent pair of saidelectrode fingers of the positive electrode at a center distance λ/2,and (iii) a floating electrode comprising a plurality of electrodefingers each situated between an adjacent one of said electrode fingersof said positive electrode and an adjacent one of said electrode fingersof said negative electrode, and offset from an intermediate positionbetween said adjacent electrode fingers of said positive and negativeelectrodes, in a direction opposite to a propagation direction of saidsurface acoustic wave in the case of the input side transducer, and inthe propagation direction of said surface acoustic wave in the case ofthe output side transducer, (iv) each of said electrode fingers of thepositive and negative electrodes and of said floating electrode of theunidirectional transducer having a width λ/12 as measured in saidpropagation direction of said surface acoustic wave, and (v) eachelectrode finger of said floating electrode of the unidirectionaltransducer being offset from said intermediate position by a distanceλ/12; and said bidirectional transducer comprises (i) a positiveelectrode comprising a plurality of sets of two electrode fingers thatare spaced from each other by a center-to-center distance λ/4, eachelectrode finger of said sets of the positive electrode having a widthλ/8 as measured in said propagation direction of said surface acousticwave, and said sets of electrode fingers of the positive electrode beingperiodically arranged at a pitch λ, and (ii) a negative electrodecomprising a plurality of sets of two electrode fingers that are spacedfrom each other by a center-to-center distance λ/4, each electrodefinger of said sets of the negative electrode having a width λ/8 asmeasured in said propagation direction of said surface acoustic wave,and each set of electrode fingers of the negative electrode beingarranged between adjacent sets of electrode fingers of the positiveelectrode.
 7. The surface acoustic wave filter device according to claim6, wherein said piezoelectric substrate comprises a quartz substrate andsaid floating electrode comprises a shortcircuit-type electrode.
 8. Thesurface acoustic wave filter device according to claim 6, wherein saidpiezoelectric substrate comprises LiTaO₃.
 9. The surface acoustic wavefilter device according to claim 6, wherein said unidirectionaltransducer is of normal electrode type and said bidirectional transduceris of a weighted electrode type.
 10. The surface acoustic wave filterdevice according to claim 9, wherein said bidirectional transducer of aweighted electrode type comprises said electrode fingers of saidpositive and negative electrodes having lengths measured in a directionperpendicular to a propagation direction of the surface acoustic wave,said lengths of the electrode fingers varying gradually in thepropagation direction of the surface acoustic wave in accordance withthe apodization-method.
 11. A surface acoustic wave filter device for aCDMA communication system, comprising:a piezoelectric substrate; aninput side transducer formed on said substrate, for exciting a surfaceacoustic wave; and an output side transducer formed on said substrate,for receiving and converting the surface acoustic wave excited by saidinput side transducer; one of said transducers comprising aunidirectional transducer and the other of said transducers comprising abidirectional transducer; said surface acoustic wave filter devicehaving a T.T.E. attenuation level not less than 30 dB, a group delaytime not greater than 0.5 μs, and an insertion loss which is not greaterthan 10 dB, wherein: said unidirectional transducer comprises (i) apositive electrode comprising a plurality of electrode fingers that areperiodically arranged at a pitch λ that is a propagation wavelength of afundamental surface acoustic wave, (ii) a negative electrode comprisinga plurality of electrode fingers that are periodically arranged at saidpitch λ and each situated with respect to an adjacent pair of saidelectrode fingers of the positive electrode at a center distance λ/2,and (iii) a floating electrode comprising a plurality of electrodefingers each situated between an adjacent one of said electrode fingersof said positive electrode and an adjacent one of said electrode fingersof said negative electrode, and offset from an intermediate positionbetween said adjacent electrode fingers of said positive and negativeelectrodes, in a direction opposite to a propagation direction of saidsurface acoustic wave in the case of the input side transducer, and inthe propagation direction of said surface acoustic wave in the case ofthe output side transducer, (iv) each of said electrode fingers of thepositive and negative electrodes and of said floating electrode of theunidirectional transducer having a width λ/12 as measured in saidpropagation direction of said surface acoustic wave, and (v) eachelectrode finger of said floating electrode of the unidirectionaltransducer being offset from said intermediate position by a distanceλ/12; and said bidirectional transducer comprises (i) a positiveelectrode comprising a plurality of sets of two electrode fingers thatare spaced from each other by a center-to-center distance λ/4, eachelectrode finger of said sets of the positive electrode having a widthλ/8 as measured in said propagation direction of said surface acousticwave, and said sets of electrode fingers of the positive electrode beingperiodically arranged at a pitch λ, and (ii) a negative electrodecomprising a plurality of sets of two electrode fingers that are spacedfrom each other by a center-to-center distance λ/4, each electrodefinger of said sets of the negative electrode having a width λ/8 asmeasured in said propagation direction of said surface acoustic wave,and each set of electrode fingers of the negative electrode beingarranged between adjacent sets of electrode fingers of the positiveelectrode.
 12. The surface acoustic wave filter device according toclaim 11, wherein said piezoelectric substrate comprises a quartzsubstrate and said floating electrode comprises a shortcircuit-typeelectrode.
 13. The surface acoustic wave filter device according toclaim 11, wherein said piezoelectric substrate comprises LiTaO₃.
 14. Thesurface acoustic wave filter device according to claim 11, wherein saidunidirectional transducer is of normal electrode type and saidbidirectional transducer is of a weighted electrode type.
 15. Thesurface acoustic wave filter device according to claim 14, wherein saidbidirectional transducer of a weighted electrode type comprises saidelectrode fingers of said positive and negative electrodes havinglengths measured in a direction perpendicular to a propagation directionof the surface acoustic wave, said lengths of the electrode fingersvarying gradually in the propagation direction of the surface acousticwave in accordance with the apodization-method.